Encode valid empty paths for empty clips

There was logic to encode a valid empty path when the clip path was empty, but this logic was ineffective because of empty line segment culling. This patch

Also includes a doc improvement for `push_layer`.

Fixes #644

vello/src/scene.rs

diff --git a/vello/src/scene.rs b/vello/src/scene.rs
index 103e9b2..d958c4b 100644
--- a/vello/src/scene.rs
+++ b/vello/src/scene.rs
@@ -66,6 +66,9 @@
     /// until the layer is popped.
     ///
     /// **However, the transforms are *not* saved or modified by the layer stack.**
+    ///
+    /// Clip layers (`blend` = [`Mix::Clip`]) should have an alpha value of 1.0.
+    /// For an opacity group with non-unity alpha, specify [`Mix::Normal`].
     pub fn push_layer(
         &mut self,
         blend: impl Into<BlendMode>,
@@ -80,8 +83,7 @@
         if !self.encoding.encode_shape(clip, true) {
             // If the layer shape is invalid, encode a valid empty path. This suppresses
             // all drawing until the layer is popped.
-            self.encoding
-                .encode_shape(&Rect::new(0.0, 0.0, 0.0, 0.0), true);
+            self.encoding.encode_empty_shape();
         } else {
             #[cfg(feature = "bump_estimate")]
             self.estimator.count_path(clip.path_elements(0.1), &t, None);

vello_encoding/src/encoding.rs

diff --git a/vello_encoding/src/encoding.rs b/vello_encoding/src/encoding.rs
index b83ec5a..d5c4684 100644
--- a/vello_encoding/src/encoding.rs
+++ b/vello_encoding/src/encoding.rs
@@ -240,6 +240,12 @@
         encoder.finish(true) != 0
     }
 
+    pub fn encode_empty_shape(&mut self) {
+        let mut encoder = self.encode_path(true);
+        encoder.empty_path();
+        encoder.finish(true);
+    }
+
     /// Encodes a path element iterator. If `is_fill` is true, all subpaths will be automatically
     /// closed. Returns true if a non-zero number of segments were encoded.
     pub fn encode_path_elements(

vello_encoding/src/path.rs

diff --git a/vello_encoding/src/path.rs b/vello_encoding/src/path.rs
index 7bb18cb..c4c2fa5 100644
--- a/vello_encoding/src/path.rs
+++ b/vello_encoding/src/path.rs
@@ -599,6 +599,15 @@
         self.n_encoded_segments += 1;
     }
 
+    /// Encodes an empty path (as placeholder for begin clip).
+    pub(crate) fn empty_path(&mut self) {
+        let coords = [0.0f32, 0., 0., 0.];
+        let bytes = bytemuck::bytes_of(&coords);
+        self.data.extend_from_slice(bytes);
+        self.tags.push(PathTag::LINE_TO_F32);
+        self.n_encoded_segments += 1;
+    }
+
     /// Closes the current subpath.
     pub fn close(&mut self) {
         match self.state {